Microsoft Suddenly Bouncing e-mails

[#1]

With a Plusnet broadband connection and a fixed IP, I have for years used my other private email addresses via that provider's SMTP servers with no problems [other than the odd bounce from microsoft or btinternet due to the reputation of certain shared servers I was hosted on (usually cleared very quickly)].

2 days ago I suddenly started getting bounces from hotmail/outlook etc addresses.  My ISP informed me that my home IP (Plusnet b/b address) was on a blocklist.  Checking with Spamhaus it was indeed, in fact, all Plusnet Broadband IPs are on a Policy Blocklist requiring that mail is sent via Plusnet.  I switched to relaying via Plusnet's relay.plus.net SMTP server and the problem went away, though the received headers aren't without some soft fails.

Either Plusnet has implemented something, or Microsoft have enforced a policy blocklist as an absolute blocklist.

Anyone else had this?  Seems to be a perfect storm of Plusnet and Microsoft.

I'd still prefer to use my hoster's SMTP servers

[#2]

Are you sending e-mails as if they were from your Plusnet account? Only outgoing e-mails that use a domain that belongs to Plusnet need be sent via Plusnet's servers - that is standard industry policy , and is enforced by SPF records that specify the addresses from which valid e-mails for that domain may originate.

[#3]

Can you please be a bit more expansive on what you are doing (how configured) and what you are doing when it goes wrong.  The description as is infers a number of potential scenarios, which might render some advice not appropriate.

1. Are you hosting a MX server on the Plusnet network?
2. Mail items related to Plusnet Email accounts should be sent via Plusnet's SMTP service relay.plus.net
3. Mail for all other email provider accounts should be sent via their  SMTP server

There are though numerous complications...

• Do your other email address forward email to the Plusnet mailbox?
• If yes, does that service implement SRS?
• Again if yes, are you trying to send using that third party email address via the Plusnet SMTP server?

The description here is odd, for if you are using the third party's SMTP server, the state of Plusnet's SMTP servers (good or bad) does not enter the discussion.

[#4]

Hi,  sorry, late response, never got the notification email

The problem occurs when I send e-mails from my own domain using a non-Plusnet hosting company's SMTP servers. It is recent (last week or so) and I have never had this particular problem before.

I have my own domain, mydomain.co.uk and the problem occurs when I use smtp.thathostingcompany.com to send mail from my own address me@mydomain.co.uk.  In that mode it has nothing to do with a Plusnet e-mail address or servers.  The SMTP server authentication I am using for the mydomain.co.uk server is the correct authentication for mydomain.co.uk and SPF and DKIM are both properly configured, and checked with that ISP, who are a small competent ISP.  E-mails to every other recipient address (non-Microsoft) are delivered ok.

My broadband IP is 31.125.83.xxx  and if you put that plus any number in xxx at https://check.spamhaus.org/  you will see at the bottom of the resultant page "This IP is listed in the Policy Blocklist (PBL)" and right at the bottom:

"Outbound Email policy of Plusnet Plc for this IP range
It is the policy of Plusnet Plc that unauthenticated email sent from this IP address should be sent out only via the designated outbound mail server allocated to Plusnet Plc customers. To find the hostname of the correct mail server to use, customers should consult the original signup documentation or contact Plusnet Plc Technical Support."

What happens if I send from me@mydomain.co.uk via mydomain.co.uk's SMTP server to a Microsoft address is I get an immediate bounce, the wording of which is not helpful.  I found out that this can be due to my broadband IP being blacklisted.  It isn't, but it is blocklisted, and Microsoft seem to be regarding the two equally?

However, if I send from me@mydomain.co.uk using Plusnet's SMTP server relay.plus.net and the correct Plusnet authentication for that (which is my Plusnet email address), then Microsoft accepts the mail. The email headers I send out still have my own IP in them at the recipient's end, but they also list the Plusnet servers in the chain.

I've had a word with my ISP and they can't see any problem with their servers, SPF/DKIM/etc at all.

So the facts are that (i) there is a Plusnet PBL, (ii) with my IP but no Plusnet server in the chain, MS block the email and (iii) still with my IP in the headers but via Plusnet servers, MS accept the email.

I don't know if this is Microsoft misconfiguring or misinterpreting the Plusnet policy as a blacklist, or what.  At least I currently have a workaround, but would prefer to get back to my own ISP's SMTP servers for mydomain.co.uk

Any advice gladly accepted

[#5]

It is correct that your Plusnet IP is in that PBL. If you are using an authenticated mail server then being in the PBL should have no effect. There is likely to be some other reason for the rejection. Are you able to share the exact wording of the unhelpful response?

Edit:  you could try putting your e-mail address into https://mxtoolbox.com/emailhealth/  to see if it reports any issues.

[#6]

Will do, and will get back to you here.

I've already seen one problem with their servers from your mxtoolbox suggestion, and have deleted the (unecessary extra) MX record. Let me wait for that to propagate.

I'll also need to switch SMTP servers back to re-obtain the error message.

Thanks

[#7]

"The Spamhaus PBL is a DNSBL database of end-user IP address ranges which should not be delivering unauthenticated SMTP email to any Internet mail server except those provided for specifically by an ISP for that customer's use. The PBL helps networks enforce their Acceptable Use Policy for dynamic and non-MTA customer IP ranges."

If the connection to the third party SMTP server is AUTHENTICATED the PBL ought not to be coming into play.

That said, the other predicate here is delivering email, not submitting email.

Which port are you using on the third party SMTP server?  Port 587 which is the correct port for submitting email from an email submission client )which is what an email application is) or port 25 which is the inter MX port for delivering email?

Though port 25 is commonly used by email submission clients (and it works most of the time), it is technically correct in that its intended use is only for MX to MX email delivery.  Strict enforcement of those principles could potentially give rise to what you are seeing.

There again, this could be a red herring!

Note if you want to use encryption on port 587 it is usually STARTTLS.  Some servers still use the deprecated (not approved) port 465 with SSL/TLS.